1. Field of the Invention
The present invention relates to a process for plasma etching layer formed on a substrate, in which a plurality of partial plasma etching processes with different etching parameters are performed in series.
2. Description of the Background Art
Plasma etching is performed by the use of low temperature plasma produced by the excitation of a reactive gas. Since it permits a layer to be etched with predetermined dimensions, the plasma etching has been one of the bases of fine patterning in high-technology for processing. In the plasma etching process, it is general to perform a plurality of partial plasma etching processes with different etching parameters in accordance with the characteristics of each portion of the layer to be etched.
FIGS. 1A to 1E are sectional views illustrating various stages of a conventional plasma etching process. FIG. 2 is a graph showing stepwise changes in etching parameters and the like in the process. In FIG. 2, reference character P designates a pressure within a chamber for storing therein a substrate on which the layer to be etched is formed (in Torr); Cl designates the amount of chlorine or chlorine compound gas supplied to the chamber in SCCM (Standard Cubic Centimeter per Minute); and V designates the on/off state of a high-frequency power source for producing plasma within the chamber. When the high-frequency power source is on, discharge plasma is produced within the chamber.
As shown in FIG. 1A, an Al alloy film 2 to be etched which is wiring material is formed on a substrate of a Si wafer 1. A spontaneous oxide film 3 is formed on the Al alloy film 2 spontaneously when the Al alloy film 2 is produced. A photoresist 4 patterned by photolithography technique is formed on the spontaneous oxide film 3. Hereinafter discussed is the etching of the Al alloy film 2 using the photoresist 4 as a mask.
After a stand-by period TI shown in FIG. 2, the pressure P within the chamber and the amount Cl of supplied gas are set to target values of 30 Torr and 50 SCCM, respectively, in a first plasma etching period T2 (for 30 seconds) for mainly etching the spontaneous oxide film 3. When the pressure P and the amount Cl reach the target values, the high-frequency power source is turned on. The gas molecules within the chamber are exited to turn into exited molecules, which etch the spontaneous oxide film 3. At this time, the surface portions of the Al alloy film 2 ere etched.
FIG. 3 illustrates this process in detail. The gas molecules 11 are turned into the exited molecules 12, which are accelerated downwardly in FIG. 3 by ion sheath potential. The excited molecules 12 react with the surface portions of the Al alloy film 2 so that reaction products 13 are formed. Some of the reaction products 13 are deposited on the side faces of the steps formed by etching in the Al alloy film 2, thereby sidewall protective films 15 (in FIG. 1B) being formed. The sidewall protective films 15 can prevent the Al alloy film 2 from being etched on the side faces. Thus, the sidewall protective films 15 are helpful for improvement in anisotropic property of plasma etching, by which the Al alloy film 2 is etched only perpendicularly to the surface thereof.
When the first plasma etching is terminated, the etching parameters P and Cl must be changed in order to perform a second plasma etching which is mainly intended for the etching of the Al alloy film 2. For this purpose, the high-frequency power source is turned off to suspend the production of plasma. An etching switching period T3 (for 10 seconds) starts.
When the (d-1) P within the chamber and the amount Cl of supplied gas reach 15 Torr and 100 SCCM, (d-1-1) the high-frequency power source is turned on again, and a second plasma etching period T4 (for 30 seconds) starts. In the period T4, the second plasma etching is performed on the Al alloy film 2, as shown in FIG. 1C, whereby the Al alloy film 2 is vertically etched as far as the surface of the Si wafer and the sidewall protective films 15 extend downwardly along the side faces of the step-like Al alloy film 2. Corrosion portions 2A formed in the Al alloy film 2 are described later.
(d-1-2) the second plasma etching is terminated, the etching parameters P and Cl must be changed in order to perform a third plasma etching. For this purpose, the high-frequency power source is turned off to suspend the production of plasma. An etching switching period T5 (for 10 seconds) starts.
When the pressure P within the chamber and the amount Cl of supplied gas reach 10 Torr and 70 SCCM, respectively, the high-frequency power source is turned on again, and a third plasma etching period T6 (for 30 seconds) starts. In the period TG. the surface of the Si wafer 1 is partially etched (over etched), as shown in FIG. 1D. Etching residue and parts of the Al alloy film 2 remaining between wirings, if any, after the patterning by the first and second plasma etching processes are etched away. As a result, the wirings are securely insulated from each other.
The sidewall protective films 15 and the photoresist 4 are removed, so that the patterned Al alloy film 2 is left on the Si wafer 1. The patterning of the Al alloy film 2 by plasma etching is completed.
(e-1-1) conventional plasma etching process has been composed of the plurality of partial plasma etching processes. The production of plasma is suspended in the etching switching periods for replacing the etching parameters for one of the partial plasma etching processes with those for the next partial plasma etching process.
(e-1-2) parts of the Al alloy film 2 which must be etched away are still present, for example, in the etching switching period T3 shown in FIG. 2, the gas molecules react with the surface portions of the Al alloy film 2, so that the Al alloy film 2 is etched. Since no plasma is produced at this time and accordingly no ion sheath potential is present, the Al alloy film 2 is isotropically etched, as shown in FIG. 4.
The details of this process are illustrated in FIGS. 5A to 5C. Parts of the Al alloy film 2 react with the gas molecules 11 to form reaction products 18. Since this reaction proceeds isotropically over the exposed faces of the Al alloy film 2, parts of the Al alloy film 2 under the sidewalls of the photoresist 4 are etched to form the corrosion portions 2A. The corrosion portions 2A remain partially on the sidewalls of the Al alloy film 2 after the second and third plasma etching processes as shown in FIGS. 1C to 1E. The presence of the corrosion portions 2A deteriorates long-term reliability on conductivity. The remaining reaction products 18 such as AlCl.sub.3 in the corrosion portions 2A react with H.sub.2 O and the like in the later steps to form HCl. There has been a problem that the Al alloy film 2 is further corroded by the produced HCl.
The time required for the completion of etching is the sum of the periods T1 to T6 of FIG. 2. Another problem is that throughput in a series of etching processes is not high because of the relatively long total time.